Electrical coupling system



g- 1931- F. CUTTING'ET AL 1,817,294

ELECTRICAL COUPLING SYSTEM Filed March 16, 1928 i'zdon 616321 29 JayLJgZor I VENTORS Patented Aug. 4, 1931 UNITED STATES PATENT GFFECE FULTON CUTTING AND JAY L. TAYLOR, OF NEW YORK, N. Y.

ELECTRICAL COUPLING SYSTEM Application filed March 16, 1928. Serial- No. 262,252.

' Our invention relates to a method of varying the coupling between two electrical circuits and is particularly applicable to circuits operating at radio frequencies.

One object of our invention is to provide a'suitable gain control or volume control for a receiver employing alternating current as a cathode heating source, in which case the usual filament rheostat cannot be convenient ly used owing to the introduction of a modulation hum.

Another object is to produce a coupling 7 device in which the coupling may be brought pure or complex.

' Fig. 4 shows a generalization of our bridge coupling device except that direct instead of inductive coupling is used;

Fig. 5 shows still another form of our variable coupling.

Referring to Fig. 1, 9 and 10 are the input connections and 11 and 12 the output terminals. L and L are the primary windings and have the same mutual inductance with L the secondary, but in the opposite sense or phase. C and C are condensers which may be equal in capacity at one point of adjustment of the variable C or both may vary simultaneously but in opposite directions (not shown) and when thus operated, would have a position of adjustment in which their capacitance would be equal.

In operation, when the condensers C and C are of equal capacity, currents of equal magnitude and phase will flow through the inductances L and L and as these windings are of opposite sense no voltage will be induced in secondary L lVhen, however, C and C are of different values of capacity currents of different magnitude and phase flow through the inductances L and L and a voltage is therefore induced in L The magnitude of this voltage may be varied by varying C, from zero which may be the position of maximum gain to a value almost equal to the capacity of C the position of minimum gain.

In reference to 2, in high frequency circuits an electrostatic coupling may prevent an exact zero being reached. An electrostatic shield between L L and L in Fig. 1 will correct this. However, unless executed in an extremely careful manner it may increase the losses in the circuit. The method shown in Fig. 2 will prevent this effect, is inexpensive and introduces no appreciable losses.

In Fig. 2, 9' and 10 are the input leads and 11 and 12 are the output terminals. C is the controlling condenser and C the equivalent of C Fig. 1. L is the equivalent of L Fig. 1. The two primaries 15 and 16 have their corresponding convolutions adjacent, but, as will be seen, are connected in the circuit so that at any given moment the current in each is of the opposite sense. It will be seen that this method of winding insures extremely close coupling between the two primary coils, and when the bridge is balanced equal currents flow through the two coils in opposite directions and on account of this almost perfect coupling no appreciable magnetic field is set up, which, of course, means there is practically no voltage drop across the coils and no electrostatic field is set up, and therefore there is substantially no electrostatic coupling to L the second ary.

Fig. 3 shows a generalization of our bridge coupling device. Z Z are two impedances having a point of common value and may be pure or complex. Z is indicated as variable but it is obvious that both may be so. L and L are the primaries coupled equally but in opposite sense to the secondary L The condition for Zero coupling is, when L and L are equal to one another and equally coupled to L that Z shall equal Z Fig. 4 shows a generalization of our bridge coupling device except that direct method of coupling is used. The input leads may be 9 and 10 and the output leads 11 and 12 but it should be understood that all these circuits are reversible in this. respect. In this variation the expression of zero coupling is equal to Fig. 5 shows a formuin which the control impedances. are inparallel with the coupling inductances.

Thesystemshown in Fig. 2 is especially applicable as a gain control for radio frequency amplifiers using. alternating current as a cathode heating source. An enormous range of control is necessary in the modern radio receiver as the received energy in the antenna circuit may vary several hundred thousand toone between the powerful local station a few blocks away and a station a thousand miles or more away of moderate power. For this reason a minimum of energy transfer which should closely approach zero should be obtainable under the conditions of great received power in the antenna. On the other hand a maximum transfer from the antenna to the following circuits should also be obtainable to make the receiver capable of satisfactory distant reception. It should also be noted that the decreased coupling, which would normally be used for local reception in a large metropolis having aconsiderable number of high power stations automatically increases the selectivity of the receiver.

As is well known theusual gain control for a radio frequency amplifier, the cathodes of whose tubes are operating on direct current, is a rheostat in the cathode circuit for reducing the cathode. heat, and hence the emission. This method is thoroughly impractical when the cathodes are heated from an alternating current source, due to the fact [iii that if the cathodeheat is reduced to anything like'the point needed for the lower ranges of sensitivity the cathode will cease to emit at all during the lower voltage values rial whether one control-impedance is varied or if both are varied oppositely.

Another advantage of our variable coupling device when used as a gain control for a radio receiver is that it will prevent the grid of the first radio frequency tube from going positive as a result of strong local signals, and thus causing modulations of signals of other frequencies.

In the above specification we have rather laid stress upon the application of our device to radio telephonic circuits. There are many other uses to which this device may be put. Therefore, we do not wish to be limited to the exactembodiment shown, except in so far as we'are limited by claims when allowed.

We claim 1. A gain control for a radio receiver including an input and an output circuit so connected in respect to the mechanisms of said receiver that substantially all the signal energy must pass into said inputcircuit and be delivered by said output circuit, said input circuit comprising two parallel branches, each of said branches consisting of an inductance and a capacitance in series, each of said inductances being equal and fixed and at least one of said capacitances being available over a range of values such that the two capacita-nces can be made electrically equal to one another, and said output circuit comprising a third inductance coupled equally to each of the aforesaid inductances in the parallel branches, but having opposite inductive relationships with each of the branch inductances respectively.

2. A varable coupling device capable of allowing energy transfer between two electrical circuits extending from a given maximum down to and including a substantial zero degree of energy transfer, including one electrical circuit comprising a single inductance coil, and another electrical circuit comprising an inductance coil with two substantially identical parallel windings having the maximum feasible physical juxtaposition so that equal and opposite currents therethrough shall produce substantially a zero electrical field thereabouts, said first coil bearing such physical relationship to said second coil that the former shall lie substantially within each and every electrical field produced by the latter, and said second coil having in series with each of its windings a capacity, at least one of said capacities being variable over a range including the value of the other capacity, the'two branch circuits comprised by each winding and the capacity connected thereunto being so connected to one another and in such respect to the energy flowing therein that the fields of these respective windings shall be in opposition to one another.

3. The method of securing areadily variable degree. of coupling, including asvirtually zero electromagnetic and electro-static coupling between two coils, which includes winding one coil with at least two physically juxtaposed and substantially identical Windings, each in a branch circuit comprising said Winding and acapacity, at least one of said capacities in the branch circuits being so yariable that the values of the several capacities may have a ratio of unity and may be made to Vary from such unity ratio to a desired degree.

FULTON CUTTING. JAY L. TAYLOR. 

